The invention concerns a calibration adapter for drilling resistance measuring devices, a calibration apparatus, and a calibratable drilling resistance measuring apparatus as well as a method for calibrated drilling resistance measurement.
Drilling resistance measurements for analysis of materials are known and are used frequently for checking the inner condition of wood objects such as trees, masts, construction lumber etc. but can also be used for other materials such as sandstone or concrete. The drilling resistance that the drilling needle penetrating the object under measurement is experiencing correlates naturally with the density of the object under measurement. Usually, in drilling resistance measurements the drilling needle is pressed with uniform feed into the object under measurement and during the drilling process the energy required therefor is measured as a function of the penetration depth of the needle. The measurement result is represented as a drilling curve, wherein the amplitudes thereof allow conclusions in regard to the drilling resistances and wherein deviations of the properties as well as density differences can be detected based thereon.
In order to obtain meaningful and reproducible measurement results, a calibration of the drilling resistance measuring device is required in order to eliminate other factors that have an influence on the measurement drilling action, for example, the condition of the employed drilling needle.
Manufacturers of drilling resistance measuring devices offer a calibration as a service, for example, in the context of an annual inspection, in order to provide the customer with a high measuring precision and reliability even after extended use of the drilling resistance measuring devices.
DE 92 17 170 U1 discloses for a density measurement by means of drilling resistance measurement a calibration by comparison with a gravimetrically determined density of a drilling core. Non-invasive methods are however preferred because they are less damaging.
EP 20 28 473 A1 discloses a method for examination and evaluation of the stability of wood masts, in particular of telephone poles or power line masts, in which the measurement characteristic lines which corresponds to the actual drilling resistance or a parameter derived therefrom for the strength of the wood or the density of the wood are determined by drilling resistance measurements on the wood mast; reference characteristic lines of the drilling resistance or a parameter derived therefrom of the lower limit for the loadability and damages, damage images, and damage classes are determined based on the comparison of the measurement characteristic lines of the drilling resistance measurements and the reference characteristic lines. In this context, a calibration of the drilling resistance values is carried out by determination of a scaling value based on mast break measurements that corresponds to the slope of a straight line through a cloud of measurement points in a diagram in which measured bending stress values are plotted against calculated bending stress values.
DE 10 2009 013 069 A1 discloses a method for examination of the condition of column-shaped or cylindrical sections of objects in which a drilling resistance measurement is combined with the recording of pressure signals at the circumference of the object section in that the drilling needle is introduced discontinuously so that a pulse tomograph is obtained. Before carrying out the method on an object to be measured, the method can be performed on a calibration body of known material quality in order to obtain comparative measurement values. When the method is then performed on the object to be measured, the obtained pulse tomograph and the obtained drilling resistance measurement can be compared with the corresponding comparative measurement results of the calibration body so that, as a function of the deviations of the obtained values, a damage of the measured object can be determined.
Based on this prior art, it is the object of the present invention to provide an apparatus with which the calibration of a measurement performed with a drilling resistance measuring device can be enabled flexibly in a simple way at many locations.